1. Field of the Invention
The present invention relates generally to transmit diversity in wireless communications, and in particular, to a method of providing cooperative diversity in a Multiple-Input Multiple-Output (MIMO) wireless network with at least one transmit antenna and at least one receive antenna.
2. Description of the Related Art
Typically, diversity gain and multiplexing gain are in a trade-off relationship. As the multiplexing gain of transmission increases, the diversity gain decreases, and vice versa. This problem can be overcome by the use of cooperative diversity.
Cooperative diversity is a technique for achieving diversity through cooperation between networks using a relay station. The relay station operates in either an Amplify-and-Forward (AF) or Decode-and-Forward (DF) mode. While the relay station amplifies a received signal prior to retransmission in the AF mode, it fully decodes the received signal, encodes the decoded signal, and then relays the coded signal in the DF mode. Due to the retransmission without decoding, the AF relay has the shortcoming that noise involved in the received signal is amplified at transmission. Although the DF relay does not suffer from the noise amplification as does the AF relay, the DF relay suffers from decoding errors that can cause errors in the coded signal. Along with a MIMO technique the cooperative diversity technique is an attractive technology for use in various, complex time and space wireless environments with limited frequency spectrum as it can provide a high data rate, an improved service quality, and a high network capacity.
However, most conventional cooperative techniques, such as the aforementioned cooperative diversity technique, were developed for implementation in a Single-Input Single-Output (SISO) network with a 1×1 antenna structure. Little is known about providing cooperative diversity for a MIMO network, and much of what is known about cooperative diversity is a mere expansion of the cooperative diversity for the SISO network. For example, a Selection Decode-and-Forward (SDF) technique was proposed in which a relay station transmits a Negative Acknowledgement (NACK) signal to a source station if a signal received from the source station is found to contain errors during decoding by the relay station.
The source station retransmits the stream to the destination source station, and the relay station relays the retransmitted signal after encoding it to the destination station if no decoding errors have occurred (e.g., see, J. N. Laneman, D. N. C. Tse, and G. W. Wornell, “Cooperative Diversity In Wireless Networks: Efficient Protocols And Outage Behavior,” IEEE Trans. Inform. Theory, vol. 50, 2004). In an application of the SDF technique to a MIMO network, if at least one error is detected from a plurality of streams received from the source station during decoding, the relay station transmits a NACK signal to the source station so that the source station retransmits all of the plurality of streams to the destination station, and in the absence of errors, the relay station relays the decoded signal after encoding it to the destination station. However, a plurality of streams characteristic of MIMO have not been individually considered in the simple expansion of the conventional cooperative diversity for MIMO networks.